U.S. Patent No. Re. 31,847, reissued Mar. 12, 1985, to Luckey discloses a storage phosphor system in which a storage phosphor is exposed to an x-ray image of an object, such as the body part of a patient, to record a latent x-ray image in the storage phosphor. The latent x-ray image is read out by stimulating the storage phosphor with relatively long wavelength stimulating radiation such as red or infrared light produced by a helium neon gas laser or diode laser. Upon stimulation, the storage phosphor releases emitted radiation of an intermediate wavelength, such as blue light, in proportion to the quantity of x-rays that were received. To produce a signal useful in electronic image processing the storage phosphor is scanned in a raster pattern by a laser beam deflected by an oscillating or rotating scanning mirror or by a rotating hologon. The emitted radiation from the storage phosphor is reflected by a mirror light collector and detected by a photodetector, such as a photomultiplier, to produce an electronic x-ray image signal. Typically the storage phosphor is translated in a page scan direction past the laser beam which is repeatedly deflected in a line scan direction perpendicular to the page scan motion of the storage phosphor to form a scanning raster pattern of a matrix of pixels.
The x-ray image signal can then be viewed as a visual image produced by a softcopy display device, such as a video display (CRT, LCD) or a hardcopy display device, such as a x-ray film printer (laser printer, CRT printer, thermal printer).
A problem exists in providing an automated technique for analyzing the photometric calibration and image quality performance characteristics of a high resolution storage phosphor reader. It is desirable that such a technique yield a comprehensive and reliable assessment of the reader performance and be compatible for use in field (hospital) and manufacturing environments. It is also desirable that the technique provide analysis of the following.
1) Exposure latitude and photometric response linearity over the 10,000:1 dynamic range of the storage phosphor.
2) Signal to Noise Ratio over the effective exposure latitudes.
3) Spatial resolution (MTF).
4) Geometric scan linearity.
5) Banding artifacts due to mechanical vibration.
6) Electronic noise artifacts.
7) Flare artifact evaluation.
Items 3, 4 and 5 are measured in both the slow (page) scan direction and the fast (line) scan direction.
Current methods of measuring photometry and image quality performance characteristics of a Storage Phosphor Reader [Computed Radiography (CR) Scanner] are based on analyzing the digitized data after it has been displayed on a separate output device. The most common method of image quality evaluation is a visual inspection of the digital output on a monitor (soft copy) or a digital laser film duplicate (hard copy). (See, for example, Photostimulable Phosphor System Acceptance Testing, by J. A. Siebert.) Visual inspections of this type are subjective and depend completely on the ability of an inspector to identify defects. In general, the analysis of the hard copy or soft copy output does not distinguish the calibration artifacts associated with the input device (scan engine) with those of the output device.
Problems therefore exist in current methods of CR image quality analysis. Such methods are, in general, non-analytic, time consuming and incapable of accurately measuring a number of important performance criteria such as spatial resolution, geometric linearity, banding noise power, signal to noise ratio, and excess scatter (or flare) artifacts.